Conductive hearing loss due to ossicular abnormalities occurs commonly due to trauma, infection, cholesteatoma, surgery to treat these diseases, and congenital anomalies, amongst other causes. Surgical reconstruction of the ossicular chain is a well-established procedure for repair of ossicular defects, but is still plagued by high failure rates, with success rates in closing the air-bone gap to less than 20 dB ranging generally only from 55%-75%. Poor hearing results in many cases can be attributed to anatomical factors and persistence or recurrence of an underlying disease process, such as tympanic membrane retraction, middle ear atelectasis, fibrosis or mucosal pathology. However, none of these fully accounts for persisting air-bone gaps following ossiculoplasty. That these factors do not fully account for the failure rates is also implied by the fact that similar results are obtained with ossicular chain reconstruction following middle ear trauma, a situation in which most of those factors are not an issue. Some degree of hearing loss can be attributable to the design of current prostheses, which do not capture all of the mechanical advantages of the normal ossicular chain. Nevertheless, it is still likely that improper fit, due to both inaccurate size, angulation, and position of the prosthesis, plays a significant role. In one series with long-term follow-up, more than 40% of failures were attributed to prosthesis size or surgeon related errors. Proper intraoperative sizing of a prosthesis is challenging, and can be affected by limited exposure and variability in the anatomic relationships of the ossicular remnants to each other or to the tympanic membrane, as well as by post-operative changes during the healing process. In particular, the medial-lateral distances between ossicular remnants, the anterior-posterior offsets, and the position of and their relationship to the tympanic membrane or neo-tympanic membrane vary widely from patient to patient in the pathologic setting, and are not always readily amenable to reconstruction with off-the-shelf prostheses.
Further, persisting conductive hearing loss following ossicular chain reconstruction is multifactorial. A significant variable in many cases is likely the underlying disease process, which may render the ear unsuitable for reconstruction over the long term. Chronic infections and associated chronic Eustachian tube dysfunction can result in stiffness of the ossicular remnants, middle ear fibrosis, middle ear atelectasis, recurrent otitis media and other factors that decrease the chances of a satisfactory hearing result, either due to intrinsic limitations to adequate sound conduction, or from displacement and/or extrusion of the prosthesis. Nevertheless, technical factors such as imprecise sizing and placement of a prosthesis also play a significant role. These data are supported by the observation that outcomes are not significantly better, if at all, for reconstruction following traumatic ossicular discontinuity, a situation in which chronic infection and Eustachian tube dysfunction are not usually a factor.
3D printed solutions have been shown to be successful adjuncts to surgical techniques. Accurately reproducing a patient's specific pathologic anatomy for preoperative planning is a common thread. Patient specific custom made anatomic models used in preoperative planning have been shown to decrease operative time and in one report to also decrease intraoperative blood loss. Additionally, models allowing for accurate surgical simulation in orthopedics and cardiovascular procedures have enhanced preoperative decision-making, improved precision and increased work efficiency. Prosthesis fabrication using 3D printed techniques is another developing field.
More particularly, three-dimensional (3D) printing has been used for a wide variety of medical applications. Custom 3D printing of an individualized ossicular prosthesis would be a potential solution for the range of anatomic variation encountered in the pathological middle ear. Custom designed prostheses could decrease the rate of post-operative prosthesis displacement, and improve the hearing outcomes, by increasing the likelihood of a proper fit. Custom printed prostheses would minimize the need for intraoperative estimates of size, and would therefore also decrease surgical time, with resultant cost savings. However, current technologies have not yet proven themselves suitable for application to the small anatomic variations found in the middle ear. Specifically, the small size of the middle ear and its ossicles present challenges both for reliable image acquisition to provide accurate data for prosthesis design, and for printing of prostheses that faithfully reproduce the measured differences.